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Canadian researchers have identified seasonal changes in the brain that may help explain why some people suffer from the winter blues.

Using a high-tech imaging machine, the scientists discovered that, in winter months, people have elevated levels of a protein that clears serotonin from the brain.

Serotonin is a neurotransmitter, or chemical messenger, instrumental in regulating mood, energy levels and appetite.

The findings, published in the journal Archives of General Psychiatry, suggest that there is less serotonin available in key parts of the brain during winter months.

"This offers an explanation for why some healthy people experience low mood and energy in the winter and why there is a regular reoccurrence of depressive episodes in the fall and winter," said Jeffrey Meyer, who led the research team at the Centre for Addiction and Mental Health in Toronto.

Mental health experts have long recognized that some people are especially prone to seasonal affective disorder, commonly known as SAD. Scientists have suspected that serotonin might be responsible for the mood swings, but they didn't know what biological mechanism might account for a change in serotonin levels.

The new research focused on a protein called serotonin transporter, which acts to remove serotonin from the synapses, or communications links between nerve cells.

"Nobody knew that [the amount of] serotonin transporter varied across the seasons," Dr. Meyer said in an interview.

He added that other factors, including seasonal differences in sunlight, probably play a role in SAD, but the latest discovery provides a new avenue of research into the common mood disorder.

For instance, many SAD patients are currently treated with light therapy - shining a light in the eyes for fixed periods of time during winter months. But the therapy doesn't work for everyone. Dr. Meyer said researchers might now be prompted to do studies to see if altering the light intensity or the duration of treatment is more effective at changing the amount of serotonin transporter in the brain.

Positive perspiration

Here's some good news for people who sweat a lot: They are apparently at a lower risk of developing exercise-induced asthma, according to a study by U.S. military researchers.

Warren Lockette, who led the team of researchers at the Naval Medical Center San Diego, noted that heavy breathing during exercise tends to dry out the airways. That means the body must have a way to rehydrate these passageways. Indeed, dry airways are more likely to become constricted or go into spasm, leaving an exerciser gasping for breath.

"It now appears that how much fluid your airways secrete could be a key determinant in protecting you from exercise-induced asthma," Dr. Lockette said.

The new research reveals that the same gene that enables people to sweat, salivate or shed tears also plays a role in hydrating the airways, according to the study published in the journal Chest. The gene basically allows water to pass across a cell membrane.

The mechanism responsible for determining sweat volume is identical to the one that controls the volume of water secreted by the airways, he said. "So if athletes sweat, drool or cry, at least they won't gasp."

Flossing for the heart

If you want to avoid a heart attack, you would be well advised to brush and floss your teeth regularly.

"The mouth is probably the dirtiest place in the human body," said Steve Kerrigan of the Royal College of Surgeons in Dublin.

Bleeding gums, from poor oral hygiene, can provide an entry point into the bloodstream for up to 700 types of bacteria. And when bacteria get into the bloodstream, he warned, it's not a good thing for the heart.

Dr. Kerrigan and scientists at the University of Bristol in England have been investigating how oral bacteria can contribute to the development of heart disease. This week, at a meeting of the Society for General Microbiology in Dublin, they presented new evidence showing that bacteria interact with platelets to form clots that can impede or even block blood flow to the heart.

"It doesn't matter how fit, slim or healthy you are; you're adding to your chances of getting heart disease by having bad teeth," said Howard Jenkinson of the University of Bristol.